Think of it as just a marketing thing, the "increased" line rental is part of the "Package price".
There use to be an option to just get Fibre without phone and it was more expensive then the base price of fibre in the package getting the two together "saved you money".
"All" the ISP's seem to sell it like this, I'm little disappointed in Zen for following the trend id prefer they just say the Fibre costs more and keep the phone cheaper. I guess its ether a marketing / physiology thing or some tax thing.
Various (Dile up) -> clara.net (Dile up) -> TELE2 (Microwave) -> ZeN (ADSL)